Device for regulating the output voltage of an amplifier by means of thermosensitive resistances



Oct. 23, 1951 c CHALHOUB 2,572,108

DEVICE FOR REGULATING THE OUTPUT VOLTAGE 0F AMPLIFIERS BY MEANS OF THERMOSENSITIVE RESISTANCES Filed June 17, 1949 Fig. 7

E JM/M I AGENTS Patented Oct. 23, 1951 DEVICE FOR REGULATING THE OUTPUT VOLTAGE OF AN AMPLIFIER BY MEANS OF THERMOSENSITIVE RESISTANCES Christian Chalhoub, Paris, France Application June 17, 1949, Serial No. 99,850 In France June 19, 1948 6 Claims.

When it is desired to maintain at a constant level the output voltage of an electronic amplifier, the method commonly used consists in rectifying this voltage and in utilizing the potential difference thus obtained for biasing of a tube having a variable slope. In addition to a certain complication, this method has the drawback of causing the value of the output voltage thus maintained constant to depend on the characteristics of a tube, and to involve a grid characteristic with a large curvature which makes it necessary, in case the harmonics of the input voltage are troublesome, to make this voltage very small.

The purpose of the present invention is to obtain, in a simple manner a regulation, substantially independent of the temperature, of the voltage level at the output of an amplifier by means of elements having a high temperature coeificient as, for example, thermo-sensitive resistances.

t depends mainly on a method for the regulation of the output voltage of an amplifier. It is a characteristic of the method that the input voltage for the amplifier is picked up at the terminals of a thermo-sensitive resistance included in a potentiometer input circuit to the terminals of which the signal to be amplified is applied. Also part of the output voltage of the amplifier is applied to an independent heating winding associated with the thermo-sensitive resistance.

This method also provides for a compensation of the efiect of a variation of the ambient temperature on the ratio of the input potentiometer by the presence of a second thermo-sensitive resistance in said potentiometer.

Another object of the present invention is the provision of circuits utilizing the above described method.

Three embodiments of the invention are described in the following specification and illustrated in the accompanying drawings wherein:

Figure 1 shows a circuit arrangement comprising only two thermo-sensitive resistances for obtaining the desired efiect:

Figure 2 shows a circuit diagram comprising an additional fixed resistance, and

Figure 3 shows a circuit arrangement for manually controlling the output level of an amplifier.

In the circuit of Figure 1, the source E supplies, through thermo-sensitive resistances I and 2, the amplifier 3, part of the output voltage of which is sent, by the network 4, to the heating element 5 of the thermo-sensitive resistance 2. The resistance elements of the two thermosensitive resistances l and 2 form a potentiometer whose middle tap a constitutes one of the input terminals of amplifier 3. To any output voltage variation of the amplifier there corresponds a compensating variation of the resistance 2, whose value varies inversely to its temperature. It is to be noted that the variations both of the source and of the amplifier are compensated.

Thus, if it is assumed that the voltage supplied by the source E decreases, the output voltage of the amplifier decreases likewise; the fraction of this voltage sent to heater 5 of the thermosensitive resistance-2 decreases likewise so that the temperature of this thermo-sensitive resistance is lowered while its resistance increases. This causes a modification of the potentiometer ratio in the direction which increases the voltage at the amplifier input terminals and restores the amplifier output to its normal level.

In case the voltage supplied by the source does not vary but the output voltage of the amplifier varies, the method of operation is the same. If the output voltage rises, the heating of the thermo-sensitive resistance 2 becomes more intense, the resistance of said thermo-resistance decreases and a smaller fraction of the input. voltage is delivered to the amplifier, the potentiometer ratio varying in an opposite direction ta the preceding example.

The thermo-sensitive resistance l is provided for securing a compensation of the effect of a. given variation of the ambient temperature on the potentiometer ratio. The following calculation will allow a better understanding of the action of this thermo-sensitive resistance.

these expressions giving the values of resistances I and 2 in terms of the absolute temperature T. In these expressions, a1 a2, 1) and b" are constant coeificients.

If the internal resistance of the source E is neglected with respect to the sum of resistances R1 and R2, the voltage U at the input of the amplifier is given by the expression 1 U 1+R1/R2 in which E designates the value of the voltage supplied by the source E.

To reduce the variations of the output voltage as functions of the ambient temperature, which becomes an additional parameter, one may equate the values assumed by the ratio Ri/RE at any predetermined limiting temperatures T1 and T2.

Let t be the increase in temperature of the thermo-sensitive resistance 2 caused by its heating element for the normal operating point. The condition is:

bl bll TIXTZ The problem is thus solved, theoretically, if a thermo-sensitive resistance l is" chosen; having the necessary temperature coefiicient. b.

This is not always possible and it is just as simple to take two thermo-sensitive resistances of the same type (b'=b") and to arrangain series, a constant resistance R1 such as resistance 6 of Figure 2.

To make the input voltages equal at temperatures T1 and T2 we must make If ROL is the valueof, R1. at temperature. T:

whence b T2 -bm- T1)T1T2 the condition becomes:

1 r 1) mm) (Tao amn),

From which one derives the value 5V1 of the resistance 6 to. insert in series'to obtain aperfoot compensation at-temper-ature T2.

It is to be noted:

1. That the internal-resistance of the generator may be included in theresistance-R'r', while the compensation by a thermo-sensitive resistance of; the coeificientb assumed theinternal resistance of. the source as being negligible compared with the-sum-R1+R2.

2. That a thermo-sensitive resistance i of any type would also give the desiredresultprovided itsv temperaturecoefficient is sufficiently high.

It isquiteobviousthat various modifications of detail may be effected withinthe scope'ofthe present invention.

Further, the term thermo-sensitive resistance used in thepresent description is tobe taken in a very wide sense; it designates any resistance whose value decreases when its temperature increases.

From the above it is seen that the present: in.- vention provides av means and method for v auto-' matically regulating the output voltagegof an amplifier, consisting in picking, up the input voltage for the amplifier at theterminals of; a resistance element having a negative temperature coefiicient and having an independent heatinggcircuit, the resistance forming part of a voltage divider supplied by the source to beamplified and in applying part of the voltage output of the amplifier to the heating circuit of theresistanceelement;

Means have also b8811 DIOVldGdifQliQOHIPGH- sating the effectof. variations of-the ambient temperature and circuits for: securing :the desired result described.

Under. such conditions; the heating circuit of thethermo-sensitive resistance ais energizediby-a current which is proportional to the output volt-- age of the amplifier, so that the input voltage varies in a direction inverse to the output voltage thus tending to keep the latter constant.

The constancy of the output voltage is of course greater as the resistance variation of the thermosensitive resistance as a function of temperature increases. Now the coefficient of resistance variation of the thermo-sensitive resistance depends, to a fairly large extent, on the temperature to which the thermo-sensitive resistance is raised, i. e.. in the present case, on the amount of current flowing through its heating circuit.

A further modification of the invention makes it possible to modify at will the output voltage of the amplifier without displacing the operating point of. the thermo-resistance selected in conformity with its construction. Thus, whatever value may be selected for the output voltage, the thermo-sensitive resistance will be able to operate at its optimum point.

Inthis modification-the internal output resistance ofthe amplifieris-high comparedtothe rcsistance in itsoutput circuit andalrnost its whole loadconsisting of. the heating circuit oithe thermo-sensitive resistance Anadjustable resistance is connectedinseries with said thermosensitiveresistanceto vary the output voltage of the amplifier.

Thismodificationis.shown in Figure 3 wherein the current source. E is connected. through a voltage divider consistingof resistance I, which is a resistance sensitive to the-ambient temperature, and of the. thermos-sensitive resistance 2, connected to the inputterminalsof amplifier 3. The last stage of; the amplifier isconstituted. by an electronic tubehaving an internal impedance much larger than its load impedances, for ex.- ample, a pentode: 9- The transformer le comprises two secondary windings i' and 'i, one of which I energizes the supply circuit and. the other of which? suppliesthe heatingcircuit 5 of the-thermo-sensitive resistancei Thetransformer. It isdesigned-in sucha manner that-for the normal inputsvoltage and the normal output. current, the. current I flowing through the heating. circuit. 5.- of the. thermo-sensitive resistance 2 raises. the latter. to: a temperature at which the resistancevariationcaused 'by avariationof the current I is large. A variable resistance S-is connected in series in the heating circuit 5 otthe thermo-sensitive resistance Under such conditions, the load of the trans former T being: assumed as consisting mainly of the heating. circuit- 0f the thermo sensitive resistance, for a voltage of'the source Eand a given amplification coeffi'cientfor the amplifier, a variation of the resistance 8- does not appreciably change the current flowing through-it, and, therefore the temperature of the thermosensitive resistance; but allows an adjustment-of the output voltage.

The resistance 8 being" set at a desired value,

a variation ofthevoltageof-E-orof the-amplification coeificient Will cause a current variation in transformer T in such adirection that the resistance of the-thermo-sensitive resistance z will be modified so as to oppose said variation and, consequently, to keep the output voltage constant. WhileI haveabove described theprinciples-of my invention in connection with specific apparatus, it is to be understood that this description i made only by wayof example andnot as a' limitation on the" scope-0f the' invention as set forth in' the objects of my inventionand the ac companying-claims."

I claim:

1. A device for automatic gain regulation of an amplifier having input and output signal terminals, comprising a voltage divider, said divider including two thermo-sensitive resistances having negative temperature ccefiicients, one of said thermo sensitive resistances comprising an insulated heater for its indirect heating, a source of signal voltage connected to said voltage divider, means connecting input terminals of said amplifier to spaced apart points on said voltage divider, a circuit for supplying current to said heater and means coupling said circuit to the output signal terminals of said amplifier, whereby the temperature of said one resistance is increased in response to an increase in amplified signal voltage at said amplifier output terminals.

2. A device according to claim 1, wherein the input terminals of said amplifier are connected across the ends of said heated resistance.

3. A device according to claim 1, wherein said one resistance which is heated has a negative temperature coeflicient of resistance of substantially greater value than the temperature coeflicient of resistance of the other thermosensitive resistance.

4. A device according to claim 1, wherein the output stage of the amplifier has a high internal resistance and wherein the circuit for supplying current to said heater includes an adjustable resistance, whereby adjustment of said adjustable resistance permits the output'signal voltage of CHRISTIAN CHALHOUB.

REFERENCESJQI'VIED The following references a of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,378,620 Chatterjea at 8.15 June 19, 1945 2,426,589 Bollman Sept. '2, 1947 2,431,207 Chatterjea et a1; Nov. 25, 1947 2,431,306 Chatterjea et a1. Nov. 25, 1947 2,471,262 Cousins May 24, 1949 

